photometric measure of the density of luminous intensity in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle

The Levels dialog box lets you correct the tonal range and color balance of an image by adjusting intensity levels of the image's shadows, midtones, and highlights. The Levels histogram serves as a visual guide for adjusting the image's key tones

Used to obscure entire layers and layer sets. By using masks, you can apply special effects without actually affecting any of the original data on that layer. After you’ve found the perfect effect, you can then apply the changes. The changes can also be discarded. Layer masks are created using the painting and selection tools.

You can obscure an entire layer or layer set, or just a selected part of it, using a layer mask. You can also edit a layer mask to add or subtract from the masked region. A layer mask is a grayscale image, so what you paint in black will be hidden, what you paint in white will show, and what you paint in gray shades will show in various levels of transparency.

Layers are like transparencies, which are clear plastic sheets of material that can be printed on. The transparencies can be printed and stacked on top of one another to form a complex picture, and single transparencies can be removed from the stack for editing or removal

There are three Lasso tools: the Lasso tool, the Polygonal Lasso tool, and the Magnetic Lasso tool. The first two choices let you draw around an object using curves and line segments, respectively, and the third lets you draw around an object and have the drawn lines snap to the object (based on calculations determined by color differences in the object and the background).

The L*a*b color model is based on the model proposed by the Commission Internationale d'Eclairage (CIE) in 1931 as an international standard for color measurement. In 1976, this model was refined and named CIE L*a*b.Lasso tools: